The hackers may be able to view what you are doing by just analysing the low-power electronic signals your laptop emits even when it is not connected to internet, researchers reported, adding that smartphones may be even more vulnerable to such spying.

By studying emissions from multiple computers, the team from the Georgia Institute of Technology in the US has developed a matrix for measuring the strength of the leaks - known technically as "side-channel signal" - to help prioritise security efforts.

"Side-channel" emissions can be measured several feet away from an operating computer using a variety of spying methods.

Each computer operation has a different potential for leaking information. The processor draws different amounts of current depending on the operation, creating fluctuations that can be measured.

Saving data to memory also requires a large amount of current, creating a "loud" operation.

"When you are executing instructions in the processor, you generate a different type of waveform than if you are doing things in memory. And there is interaction between the two," said Alenka Zajic, assistant professor at Georgia Tech.

To measure the vulnerability, Zajic and the team developed a metric known as "signal available to attacker" (SAVAT), which is a measure of the strength of the signal emitted.

They measured the level of SAVAT for 11 different instructions executed on three different laptops and found the largest signals when the processors accessed off-chip memory.

It is not really possible to eliminate all "side-channel signal".

“The trick is to make those signals weak so potential attackers would have to use larger antennas and utilise time-consuming signal analyses," Zajic added.

The researchers are also now studying smartphones, whose compact design and large differential between idle and in-use power may make them more vulnerable.

The research was presented at the 47th annual IEEE/ACM international symposium on micro-architecture in Cambridge recently.